Windshield wiper system

ABSTRACT

A windshield wiper system (10) which includes an electrical drive motor (12) that makes available a driving moment for at least one windshield wiper (16) performing a back and forth movement on a pane to be cleaned. A signal processing arrangement (20) is provided which, by way of an electrical switching stage (18), controls or regulates the rotation rate of the drive motor (12) to a constant value.

STATE OF THE ART

The invention is based on a windshield wiper system of the typeincluding at least one electrical drive motor that makes available adriving moment for at least one windshield wiper performing a back andforth movement on a pane to be cleaned. A windshield wiper driven by anelectric motor and performing a back and forth movement on a pane to becleaned has a varying requirement for torque during a wiping cycle, forexample, with this requirement being a function of the state of the paneto be cleaned and of the position of the windshield wiper. A greatchange in the torque requirement occurs in the reversal positions of thewindshield wiper since there the required driving moment temporarilytakes on the value of zero. The different torque requirement during awiping cycle leads to differences in load on the drive motor whichinfluence the noise development of the motor. The short-term absence ofdriving torque in the reversal positions, particularly in theincreasingly employed small, high speed electric motors, leads to atemporary increase in the rate of rotation which is considered to beparticularly annoying.

It is the object of the invention to reduce the noise development ofsuch windshield wiper systems.

SUMMARY OF THE INVENTION

The above object is generally achieved according to the presentinvention by a windshield wiper system including at least one electricaldrive motor that makes available a driving moment for at least onewindshield wiper performing a back and forth movement on a pane to becleaned, and a signal processing arrangement which, via an electricalswitching stage, controls the rotation rate of the motor to a constantvalue, with the switching stage determining the operating voltage (U₊)of the motor as a function of the current (I_(M)) flowing through themotor.

ADVANTAGES OF THE INVENTION

Regulation of the rotation rate as well as the control of the rotationrate of the electric windshield wiper drive motor according to theinvention reduce the noise development of the drive. The differencebetween control and regulation of the rotation rate lies in thatregulation includes means for detecting the rate of rotation of themotor so as to put out a signal to a signal processing arrangement. Acontrol is able to operate without this report of the rate of rotationand can therefore be realized more economically. The regulation of therate of rotation of the motor permits the input of a desired rotationrate value while the control keeps the rate of rotation essentiallyconstant, with the precise value of the rate of rotation being ofsubordinate significance. Significant for the invention is the avoidanceof greater changes in the rate of rotation during wiping operation.

Keeping the rate of rotation of the drive motor constant reduces,particularly in the reversal positions of the windshield wiper, thenoise from the drive. A change in the noise, to which the human earreacts with particular sensitivity, is substantially avoided.

Another advantage of regulation or control of the rate of rotation ofthe electric drive motor results from the possibility of providingdifferent wiping speeds which are set electronically with the aid of thesignal processing arrangement. The action on the motor practiced in thepast, for example, the use of a collector direct current motor havingthree instead of two brushes, is no longer required. The omission of thethird brush further reduces noise development and increases theefficiency of the drive. The electronic control reduces the costs forinterference suppression for the electric motor which might have to beimplemented.

Advantageous modifications and improvements of the basic systemaccording to the inventor are disclosed.

A first advantageous modification provides for the derivation of a valuethat is proportional to the rate of rotation of the electric motor fromthe electromotor countervoltage of the motor. This voltage can bemeasured at a motor that is operated as a generator. Generator operationduring windshield wiper operation can be cyclically repeated by usingpulsed operation in which the drive motor is connected with theoperating voltage with a defined pulse to pause ratio. Thecountervoltage of the electric motor can be measured by the signalprocessing arrangement during the pulse pauses and can be converted intoa corresponding rotation rate signal.

An advantageous possibility for at least controlling the rate ofrotation of the electric motor is given by the current flowing throughthe motor. This possibility is based on the fact that the driving momentis approximately proportional to the motor current. The signalprocessing arrangement puts out the operating voltage of the electricmotor as a function of the desired rotation rate and the current flowingthrough the motor as well as a function of the known internal resistanceof the electric motor.

An advantageous feature of the system according to the invention relatesto the measuring of the current flowing through the electric motor withthe aid of an electrical resistance that is disposed in the immediatevicinity of the electric motor. With this measure it is possible tocorrect the influence of the operating temperature on the motorcharacteristics simultaneously with regulating or controlling therotation rate of the motor. Without a correction, an increase intemperature would lead to a drop in the rotation rate during controlledoperation. This measure can further be utilized to realize a protectionagainst excess temperatures. Instead of detecting the temperature by wayof a measuring resistor for the current flowing through the motor, atemperature sensor may be provided that is associated with the motor andemits an output signal to the signal processing arrangement.

An advantageous modification provides for a redundancy of the motorcurrent supply which, in normal operation, is effected by a switchingstage that is actuated by the signal processing arrangement and includesthe electrical power component for the motor. The additional switchingstage required for this purpose is able to detect, for example, theoperating voltage present at the drive motor and, if a predeterminedvalue is not reached, take over the energy supply. The predeterminablevalue of the switching threshold preferably lies slightly below thevalue of the minimum operating voltage of the electric motor providedfor the lowest wiping speed.

Further advantageous modifications and improvements will become evidentfrom further dependent claims in conjunction with the description below.

BRIEF DESCRIPTION OF THE DRAWINGS

The sole drawing figure depicts a block circuit diagram for a windshieldwiper system according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Windshield wiper system 10 includes an electrical drive motor 12 whichactuates a windshield wiper 16 by way of a drive mechanism 14. To supplymotor 12 with energy, a switching stage 18 is provided which receives acontrol signal from a signal processing arrangement 20. Switching stage18 lies at a supply voltage U_(B) and makes available an operatingvoltage U₊ for motor 12. The signal processing arrangement 20 as well asan additional switching stage 22 are also connected to supply voltageU_(B). The line which is connected to the operating voltage U₊ of themotor 12, is connected, in addition to motor 12, also with signalprocessing arrangement 20 and the additional switching stage 22.

The current I_(M) flowing through motor 12 is determined in arrangement20 with the aid of the voltage drop across a resistor 24. Resistor 24 isconnected to ground 26. Motor 12 has an associated temperature sensor 28which feeds a signal to arrangement 20. Means are also provided fordirectly detecting the rate of rotation of motor 12. These means areshown in the drawing by way of a line leading from the motor shaft toarrangement 20 for transmitting the rotation rate signal. Signalprocessing arrangement 20 is further connected with an operator element30.

The windshield wiper system 10 according to the invention operates asfollows:

By way of a drive mechanism 14, the electric drive motor 12 actuateswindshield wiper 16 which performs a back and forth movement on a panethat is not shown in the drawing. Instead of a motor 12 and a windshieldwiper 16, a plurality of motors 12 and a plurality of windshield wipers16 may also be provided. In conjunction with switching stage 18, signalprocessing arrangement 20 is initially provided to keep the rotationrate of motor 12 constant during the wiping process. Keeping therotation rate constant results in a reduction in noise and particularlyin a reduction of changes in the noise. Changes in the rotation rateresult from the changing torque during a wiping cycle. The greatestamount of change occurs in the reversal positions of windshield wiper 16where the required driving torque briefly becomes zero. The rotationrate of the briefly relieved motor 12 could then briefly rise. Thesechanges in the rotation rate of the electric drive motor 12 areprevented by regulating or at least controlling the rate of rotation.

As the simplest measure for regulating the rotation rate, means areprovided to detect the rotation rate of motor 12. These means pass theactual rotation rate to signal processing arrangement 20. Arrangement 20compares the actual rotation rate with a predeterminable desired valueand actuates switching stage 18 in such a way that the desired rotationrate is realized. Instead of controlling the operating voltage U₊, it isalso possible to change the pulse to pause ratio that occurs in theso-called pulsed operation. In this pulsed operation the motor isconnected with a comparatively high frequency pulse-shaped clock signalat operating voltage U_(B). By way of the pulse to pause ratio, theaverage electrical energy is predetermined. Compared to an analog inputof U₊, pulsed operation increases the electrical efficiency of the wipersystem as well as the torque to be put out at low rotation rates.

Instead of a special rotation rate sensor, the electromotorcountervoltage occurring in motor 12 may also be employed as a measurefor the rotation rate of the motor; it can be measured during generatoroperation of motor 12. In pulsed operation, generator operation occursduring the pulse pause. For that reason, the line in which operatingvoltage U₊ is present is shown in the drawing figure as being connectedwith signal processing arrangement 20. The electromotor countervoltageinduced in motor 12 during generator operation is an accurate measurefor the rate of rotation. It is therefore possible to regulate therotation rate to a predetermined value even without a special rotationrate sensor.

A simple possibility for at least controlling the motor rotation rate isgiven by measuring the current flowing through motor 12. The rotationrate of motor 12 is proportional to the internal motor voltage U_(M).Likewise, the moment M put out by motor 12 is at least approximatelyproportional to the current I_(M) flowing through motor 12. It is thuspossible to change the motor characteristic, which represents therotation rate as a function of torque, to the electrical equivalentvalues of motor 12 and this results in the characteristic U_(M) =U_(B)-Ri×I_(M), where Ri is the internal resistance of the motor which can bedetermined from U_(B) /I_(K). The short-circuit current of the motorwhich may be given by the manufacturer or can be determined by way of ameasurement is marked I_(K). In order to obtain a constant motorrotation rate independent of the driving torque, U_(M) must be keptconstant. The operating voltage U₊ of motor 12 must followcorrespondingly. The operating voltage U₊ must meet the condition U₊=U_(M) +(U_(B) /I_(K) ×I_(M)). Different wiping speeds and consequentlydifferent desired rotation rates can be predetermined by differentvalues for U_(M). If the desired rotation rate is fixed, the operatingvoltage U₊ to be given for motor 12 is then dependent only on the motorcurrent I_(M) to be determined. The rotation rate can be adjusted aslong as the value to be given for operating voltage U₊ does not exceedthe value of the available operating voltage U_(B). A desired rotationrate cannot be precisely predetermined in controlled operation. Theapproximate value of the desired rotation rate as a function of themeasured current I_(M) can be determined experimentally. Significant isthat the rotation rate is kept constant during controlled operation.

An advantageous feature relates to the arrangement of a resistor 24 inthe immediate vicinity of the motor. With this measure it is possible tocorrect the temperature influence on the electrical parameters of theelectric drive motor 12 so as to thermally couple the resistor to themotor as indicated by the dashed line 25 existing therebetween in signalprocessing arrangement 20. Particularly advantageous is the realizationof resistor 24 as a correspondingly calculated copper conductor on aprinted circuit. The temperature coefficients of the armature winding ofelectric motor 12 and of resistor 24 are thus identical and permit aparticularly simple correction of the temperature influence withoutfurther measures. Additionally, the operating temperature can bemonitored by means of a separate temperature sensor 28 which is arrangeddirectly adjacent to the motor. The monitoring of the operatingtemperature of motor 12 constitutes an effective protection againstthermal overloads which may occur, for example, upon a short-circuit orif the drive mechanism 14 is blocked.

If switching stage 18 which includes electronic power components ismalfunctioning, the additional switching stage 22 takes over the energysupply for electrical drive motor 12 and thus permits at least themaintenance of emergency operation of the windshield wiper system 10.The additional switching stage 22 includes, for example, a voltagecomparison circuit which compares operating voltage U₊ with respect toground 26. If the measured value lies below the operating voltage U₊ tobe given for the smallest pre-selected rotation rate corresponding tothe slowest wiping speed of windshield wiper 16, a defect in switchingstage 18 can be assumed to exist. Without further measures, additionalswitching stage 22 is able to set an operating voltage U₊ that almostcorresponds to the required operating voltage U₊ for operation at theslowest wiping speed to be set. With a slightly greater amount ofelectronic circuitry, it is also possible to completely replaceswitching stage 18 by the additional switching stage 22 which then isalso able to make available the full operating voltage U₊ for operationat maximum wiping speed.

Operating element 30 is intended for switching the windshield wipersystem 10 on and off and for preselecting a wiping speed or forintermittent operation. Operating element 30 is preferably connectedwith signal processing arrangement 20 which controls switching stage 18according to the preselected operating mode.

I claim:
 1. A windshield wiper system comprising at least one electricaldrive motor that makes available a driving moment for at least onewindshield wiper performing a back and forth movement on a pane to becleaned, and a signal processing arrangement which by way of anelectrical switching stage controls the rotation rate of the motor to aconstant value, said switching stage determining the operating voltageof the motor as a function of the current flowing through the motor. 2.A system according to claim 1, wherein the current flowing through themotor is determined by way of the voltage drop occurring across aresistor.
 3. A system according to claim 2, wherein the resistor isthermally coupled with the electrical drive motor.
 4. A system accordingto claim 1, further comprising means for detecting the operatingtemperature of the motor so as to put out a signal to the signalprocessing arrangement.
 5. A system according to claim 1, furthercomprising an operating element for preselecting at least two differentwiping speeds and the operating mode of the windshield wiper system. 6.A system according to claim 1, further comprising an additionalswitching stage for permitting emergency operation of the windshieldwiper system upon a malfunction of the first-mentioned switching stage.